1. Field of the Invention
Embodiments of this invention relate to a movement-adaptive noise reduction apparatus wherein, for example, an image signal in a unit of a frame is stored into a memory and then mixed with a next image signal at a predetermined mixture ratio which is adaptively varied in response to a movement of an image. The embodiments of the present invention further relate to an image signal processing apparatus having a movement-adaptive noise reduction circuit and an amplifier provided at a preceding stage to the movement-adaptive noise reduction circuit, and an image inputting processing apparatus such as a camera apparatus which includes an image pickup device and so forth in addition to the image signal processing apparatus. The embodiments of the present invention relate also to a movement-adaptive noise reduction method.
2. Description of the Related Art
An image signal or a video signal includes information of a color and a luminance in a time series in a unit of a screen image or more finely in a unit of a pixel. Generally, a display period or a signal interval of one screen image is called frame. Further, an image of this basic unit is hereinafter referred to as frame image or 1F image.
An image, particularly of a dynamic picture, has a high correlation between frames, and a movement-adaptive noise reduction method which utilizes this correlation to remove noise while preventing blurring of a dynamic picture is known and disclosed, for example, in Japanese Patent Laid-Open No. Hei 05-328174 (hereinafter referred to as Patent Document 1) and Japanese Patent Laid-Open No. Hei 06-225178 (hereinafter referred to as Patent Document 2).
According to the method, an image preceding by one screen image is stored into a frame memory and is mixed at a predetermined mixture ratio with an image of a succeeding frame. Successive images have a high correlation, and even in a dynamic image which exhibits a large movement, successive images have some correlation. In contrast, noise components have little correlation. Therefore, if images of two successive frames which include noise are mixed, then the noise components partly cancel each other to reduce the noise level. This image mixture and storage into a frame memory after the mixture are cyclically executed along a feedback path including the frame memory. Consequently, for example, if the mixture ratio is 1:1 (mixture coefficient K=0.5) and the noise level decreases to one half for each one frame, then the noise can be suppressed sufficiently in ten and several frames to approximately several tens of frames. Such a process as just described is called frame-cyclic noise reduction filtering or three-dimensional noise reduction (3DNR).
However, in the case of a dynamic picture, if the mixture coefficient K is set to a high value, then an after-image appears. Therefore, it is necessary to adaptively control the mixture coefficient K depending upon whether a dynamic picture or a still picture is handled.
In order to satisfy the demand just described, each of the circuits disclosed in Patent Documents 1 and 2 includes a subtractor and a movement decision section. The subtractor calculates the difference between an image signal before mixture and another image signal in a frame memory in order to extract a movement component. The movement decision section carries out movement decision based on the movement component outputted from the subtractor and controls the mixture coefficient K in response to a result of the decision.
Further, since generation of a movement coefficient by the circuit is carried out by calculation of the difference between two successive image signals which have a high correction but have different noise levels, the movement component inevitably includes noise components. In the movement decision, it is decided that, for example, where the movement component is higher than a threshold value, it is decided that the images are a dynamic picture, but where the movement component is lower, the images are a still picture. Therefore, it is desirable to remove noise components as much as possible from a movement component.
The circuits disclosed in Patent Documents 1 and 2 are common in that they include a low-pass filter (LPF) for removing noise components.
In the circuit disclosed in Patent Document 1, low-pass filtering is applied to a movement component outputted from the subtractor.
On the other hand, in the circuit disclosed in Patent Document 2, low-pass filtering is applied to a current image signal before inputted to the subtractor.